Pocket
IGCSE Basic Chemistry Facts
A
Selection of Chemical Tests
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Hydrogen: Insert
lighted splint into test tube of hydrogen.
Listen out for a “squeaky pop”
Oxygen: Insert a
glowing splint into a test tube of oxygen.
Glowing splint should relight
Water: Add a
spatula of white anhydrous copper sulphate to water. Colour changes from white to blue
Chlorine: Add damp
litmus paper. Chlorine will bleach the
paper (lose its colour)
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IDENTIFYING
POSITIVE IONS
Flame
Tests
Flame tests are a quick way of identifying
positiveions.
Place a few crystals of the unknown
solid into the blue Bunsen flame and observe the immediate colour of the
flame.
Ion Colour
of flame
_____________________________________
lithium, Li+ scarlet (red)
sodium, Na+ persistent
yellow/orange
potassium, K+ lilac (pink/purple)
calcium, Ca2+ brick red
The Sodium Hydroxide
Test
In a test
tube add a few drops of sodium hydroxide solution to a sample of the unknown
solution
Metal ion
being tested Result
(after adding
sodium
hydroxide solution)
____________________________________________________________________
copper Cu2+ blue
precipitate (ppt)
iron (II) Fe2+ green
ppt
iron (III)
Fe3+ brown
ppt
___________________________________________________________________________________
Please note:
“ppt” is the abbreviation for “precipitate”
Testing for ammonium
ions, NH4+
dilute sodium hydroxide solution
In a test tube add a few cm3
of NaOH(aq) to the unknown sample and
warm gently
POSITIVE RESULT: ammonia gas is produced which can be tested with damp red litmus paper,
which turns blue because ammonia is an alkaline gas.
IDENTIFYING
NEGATIVE IONS
1. Testing for
carbonate ions, CO32-
dilute hydrochloric acid
- Add some dilute hydrochloric acid to the unknown compound. It should fizz.
- The gas given off is CO2 which can be confirmed by bubbling it through limewater.
POSITIVE RESULT:
Limewater turns cloudy
2. Testing for halide
ions – THE SILVER NITRATE TEST
A “halide”
ion means "chloride", "bromide" or "iodide".
silver nitrate solution
- To unknown solution acidify with a few drops of nitric acid
- Add a 10 drops of drops silver nitrate solution
Halide ion POSITIVE RESULT_________________________________________________________________________________
chloride white
ppt
bromide pale
yellow ppt
iodide yellow
ppt
_________________________________________________________________________________
3. Testing for sulfate, SO42- –THE BARIUM CHLORIDE
TEST
barium chloride solution
- To unknown solution acidify with a few drops of dilute hydrochloric acid
- Add a few drops of drops barium chloride solution
POSITIVE RESULT: white ppt
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Solubility Rules
These solubility rules
may help you to narrow down your enquiry:
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Ionic
Compounds
Atomic
and Molecular Ions(an ion is a
charged atom or a charged molecule)
Many compounds
are ionic. The table below gives the
formulae of some common ions met in GCSE chemistry. As well as being either positively or
negatively charged, ions can be either atomic
or molecular.
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Working
out the formula of an ionic compound
In the table opposite, any one of the positive
ions will attract a negative ion.
For example, a calcium ion will attract a hydroxide ion. However, the formula of calcium hydroxide
is not “CaOH”. If it was,
then it would still have some positive charge left over. We also know that calcium hydroxide is
electrically neutral because you don’t get an electric shock when you touch
it!The reason why youdon’t get an electric shock from materials made from
ions is because the material must have as much positive charge as negative
charge. The charges must be
balanced. Therefore, the formula of magnesium
hydroxide is Mg(OH)2
Please note: this table only applies to ionic compounds. It cannot be used to work out the formula
of covalent compounds(see later)
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Table showing the names and formulae of common
ions.
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Positive ions
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Formula
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Negative ions
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Formula
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Hydrogen
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H+
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Hydride
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H-
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Lithium
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Li+
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Chloride
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Cl-
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Sodium
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Na+
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Bromide
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Br-
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Potassium
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K+
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Iodide
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I-
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Ammonium
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NH4+
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Hydroxide
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OH-
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Calcium
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Ca2+
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nitrate
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NO3-
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Magnesium
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Mg2+
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Oxide
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O2-
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Copper
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Cu2+
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Sulfide
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S2-
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Iron (II)
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Fe2+
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Sulfate
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SO42-
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Iron (III)
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Fe3+
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Carbonate
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CO32-
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Aluminium
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Al3+
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Some examplesof
formulae of ionic compounds:
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Examples of
chlorides
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Examples of
hydroxides
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Examples of
nitrates
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Examples of
oxides
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Examples of
sulfates
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Examples of
carbonates
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|
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NaCl
CaCl2
AlCl3
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NaOH
Ca(OH)2
Al(OH)3
NH4OH
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NaNO3
Ca(NO3)2
Al(NO3)3
NH4NO3
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Na2O
CaO
Al2O3
Ammonium oxide doesn’t exist
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Na2SO4
CaSO4
Al2(SO4)3
(NH4)2SO4
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Na2CO3
CaCO3
Al2(CO3)3
(NH4)2CO3
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“ammonium nitrate” – an important fertiliser
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“aluminium carbonate”
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Q. Which three columns contain the
formulae of salts?
The remaining columns contain the formulae of bases. Remember, a base is a substance that
neutralises an acid to make salt + water.
Hydroxides are a particular kind of base called alkalis. Alkalis contain hydroxide ions. Carbonates are a special kind of base which
produce CO2as well as salt + water, when neutralised.
What
about the formulae of covalentsubstances?
Covalent elements and compounds
achieve stable electron configurations by sharing their electrons so it’s
possible to work out the formula by drawing dot-cross diagrams or using a table
of valencies (see Y10 “Back to Basics” topic). But actually, it’s quicker just to learn
them!
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Formulae of
common molecules
hydrogen, H2 oxygen, O2 nitrogen, N2 chlorine, Cl2 bromine, Br2
sulphur, S (actually
it’s S8 but that’s A-Level) water,
H2O ammonia,
NH3 carbon
dioxide, CO2
carbon monoxide,
CO sulphur
dioxide, SO2 nitrogen
dioxide, NO2
hydrochloric
acid, HCl sulphuric
acid, H2SO4 nitric
acid, HNO3
methane, CH4, ethane, C2H6 propane, C3H8 butane, C4H10 pentane, C5H12
ethene, C2H4 propene, C3H6 butene, C4H8 pentene, C5H10 hexene, C6H12
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Pocket
IGCSE Basic Chemistry Facts
Elements and compounds
- An element is made of its own type of atom. An element is a substance that can’t be broken down into any simpler
- Compounds are made of two or more elements, which are chemically combined.
- Molecules are particles made of two or more atoms covalently bonded together. Water is a compound made of molecules (H2O). Oxygen is an element made of molecules (O2) but diamond is an element made of atoms (C). You might think that diamond is made of molecules - after all, it’s made of more than one atom that are all covalently bonded. However, diamond is not made of a specified number of atoms (unlike water which is always made of 3 atoms). Diamond is a crystal or it can be described as a giant covalent structure of atoms.
Mixtures
A mixture contains two or more substances that
can be separated easily. Q.
Can a mixture be pure?
Can
you describe when and how you would use the following purification/Separating
techniques?
- Filtration (separates a solid from a liquid),
- Evaporation (separates a solid from a solution),
- Distillation (separates a pure liquid from a solution),
·
Chromatography (separates colours from a dye),
Crystallisation: this is the best way of getting a
pure solid from a solution – you only partly evaporate the solution (to
make it saturated), leave to cool, so crystals form. Then you filter off the crystals, wash
them &leave them to dry.
Acids, Alkalis and Salts
The
pH scale (1-14). Universal indicator turns red when pH is 1, green when
pH is 7 and purple when pH is 14. Do you know the other colours for the other
pHs?
Acid
+ alkali/base → salt + water the
reaction is called neutralisation.
e.g.
hydrochloric acid + calcium hydroxide → calcium chloride + water.
Acid + carbonate → salt + water
+ carbon dioxide,
e.g.
sulphuric acid + magnesium carbonate → magnesium sulphate + water +
carbon dioxide
Acid + reactive metal → salt +
hydrogen
this
is notneutralisation because water isn’t made.
In fact, it’s a displacement or a redox reaction
e.g.
zinc + nitric acid → zinc nitrate
+ hydrogen {Q. Do copper, silver & gold react with
acids? Why not?}
The salt can be extracted by
evaporation or by crystallisation
What’s the difference
between a base and an alkali?A base is any
chemical that neutralises an acid to make
salt + water. Alkalis are bases that contain “hydroxide” ions
(e.g. calcium hydroxide) and are soluble.
ACIDS
IN MORE DETAIL: All acids contain hydrogen ions, H+(aq). The evidence for this is that acids conduct
electricity and hydrogen gas is always produced at the negative electrode. Hydrochloric acidis an aqueous solution containing
H+(aq) ionsas well as Cl-(aq) ions. But hydrogen chloride gas, HCl(g), or
hydrogen chloride dissolved in a non-aqueous solvent such as methybenzene, does
not conduct electricity - so they cannot contain hydrogen ions and are not
acidic. In summary, hydrogen chloride is
made of molecules. Hydrogen chloride
only becomes acidic when it is added to water.
This is because hydrogen chloride molecules react with water and break
up into H+and Cl-ions, becoming hydrochloric acid, HCl(aq)
Reactivity Seriespotassium sodium magnesium aluminium CARBON zinc iron
HYDROGEN copper silver gold
- Displacement reactions – a more reactive metal displaces a less reactive metal,
e.g. magnesium
+ copper sulphate → magnesium sulphate + copper
- Use of carbon (e.g. in a blast furnace) to extract metals from their oxides:
e.g. iron
oxide +
carbon ® iron + carbon dioxide
- Electrolysis is used to extract reactive metals (metals which are more reactive than carbon) such as sodium.
·
Rustingof iron is a type of corrosion
& requires oxygen and water. It can
be prevented by stopping either air or water from getting to the iron (e.g.
paint) or by the use of sacrificial protection(e.g. galvanising with zinc)
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